Week 3 Flashcards
What is the range of typical physiological strains of bone?
50-3000 microstrain (0.005-0.3%)
Explain:
- Anisotropic
- Orthotropic
- (transversally) isotropic
- Anisotropic; material properties are not the same in every direction.
- Orthotropic: material properties depend on the direction you measure (3 directions, 9 independent elastic constants) > cancellous bone.
- (Transversally) isotropic: material properties do not depend on the direction (5 independent elastic constants). Transversally isotropic: symmetric on 1 axis > cortical bone.
Bone is stronger in compression/ expansion.
Bone is stronger in compression.
What are the bone ELASTIC constants for:
- Cortical bone
- Cancellous bone
What are the bone ELASTIC constants for: 1. Cortical bone Elong = +- 20 GPa Etrans = +- 13 GPa G= +- 1- GPa v= +- 0.3 2. Cancellous bone Elong, Etran, G, and v depend on density & fabric tensor.
What are the bone STRENGTH constants for:
- Cortical bone
- Cancellous bone
What are the bone ELASTIC constants for: 1. Cortical bone Stress, long yield= +- 200 MPa Stress, trans yield = +- 130 MPa Strain, yield= +- 0.5%
- Cancellous bone
Stress, yield= depend on density & fabric tensor.
Strain, yield= +- 0.5%
What are the 2 methods how we translate an external force to local stresses for complex shapes?
What are the 2 methods how we translate an external force to local stresses for complex shapes?
- Beam theory
- Finite element modelling
Explain beam theory.
It assumes bones can be considered as prismatic beams. The moment of the area only depends on the shape of the cross-section.
- Axial loading generates axial normal stresses.
- Bending generates axial normal stresses.
- Torsion generates shear stresses.
By measuring the area (A) + second moment of area (I) + the torsion polar 2nd moment of area J —> it becomes possible to translate external moments/forces to local stresses.
If the material yield stress is known, it’s possible to calculate the failure load/moment.
Why do our bones have a specific cross-sectional shape?
It determines the ratio compression strength to bending/ torsion strength. This is determined by the bone mass vs. 2nd moments of area.
Examples:
- Alligator: heavy bones to keep them underwater. Very thick bone, small inside.
- Camel: similar to human.
- Specific bird: air-filled bones. Very small bone, large inside. Birds have a lot of bending but not loading.
Explain Finite Element analysis/method (FEM).
An engineering technique for numerically solving differential equations to calculate mechanical properties and deformation of structures under loading. To solve a problem, the FEM subdivides a large system into smaller, simpler parts that are called finite elements.
What are the input & output parameters of FE models?
Input: bone geometry, external loading condition, bone material properties.
Output: stiffness, strength, deformation, stress/strain.
What are 2 limitations of FE models of bone?
Models do not account for bone microstructure, trabecular bone is ‘smeared out’ to a continuum.
Require empirical density-modulus/ strength relations.
What is endochondral ossification?
Endochondral ossification=
- primary and secondary center of ossification. Bone develops by replacing hyaline cartilage. Cartilage does not become bone. Instead, cartilage serves as a template to be completely replaced by new bone. It stops when epiphysis grows together with diaphysis (fusion), disappearance of epiphyseal plate. Endochondral ossification takes much longer than intramembranous ossification.
- Bones: at the base of the skull and long bones.
What is intramembraneous ossification?
Intramembraneous ossification=
- compact and spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue.
- Bones: the flat bones of the face, most of the cranial bones, and the clavicle (collarbones) > schedel, onderkaak, sleutelbeen.
Give characteristics of periosteum
The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces of long bones.
Dense, irregular connective tissue on the outside of bones.
- Outer fibrous layer: fibroblasts
- Inner osteoenic layer: cambium, progenitor cells.
Helps in the attachment of tissues.
What is ossification/ osteogenesis? And what are the 2 forms?
The natural process of bone formation. |There are 2 main forms: endochondral & intramembraneous.
